It is well known in the chemical engineering and pharmaceutical industries to provide pressure relief devices for protecting pressure systems from over pressurisation. One such pressure relief device is a bursting disc, for example as disclosed in WO 03/031853. When the pressure at one side of the disc rises above a predetermined design pressure, the disc ruptures thereby releasing pressure from the system. One such typical application is on reaction vessels and chambers in the chemical, pharmaceutical and food industries.
It is also known in the art to provide a bursting disc in an over-pressure detector. WO 2005/054731 discloses a bursting disc type over-pressure detector having a bursting disc clamped at its flange between inlet and outlet pipe members. At the vent side of the device in WO 2005/054731 is mounted a magnet, its movement sensed by a non-invasive sensor. When the pressure at one side of the disc rises above a predetermined design pressure, the disc ruptures thereby releasing pressure from the system and moving the magnet relative to the sensor. The signal produced by the sensor changes, signalling that the bursting disc has ruptured.
Bursting discs are often used in equipment housing processes that use corrosive materials, e.g. sulphuric and nitric acids, chromic acid, phosphoric acid, trichloroethylene, etc. It is known that bursting discs can be made from corrosion-resistant materials where they are intended to protect a process utilising corrosive chemicals. The materials of construction of such bursting discs are generally selected on a case-by-case basis depending on the particular applications.
To manufacture a bursting disc for a particular application, the specific material (both material type and thickness) must be purchased and the manufacturing process for the disc must be adjusted to suit. Discs for arduous chemical environments are made of a solid material, which is chemically compatible with the process chemistry. As these materials are often of a very specialised nature they are usually very expensive and available only in certain forms, which may not be suitable for bursting disc manufacture. These factors increase the delivery lead-time and costs of the bursting disc.
A cheaper alternative to making bursting discs from expensive corrosion resistant materials is to provide a thin coating on the process contact face of the disc, in order to provide a barrier between the chemical process and the structure of the disc. GB-A-2307654 shows an example of a deposited and sintered PTFE coating. This coating is thin and flexible and conforms to the surface of the bursting disc. It provides no mechanical strength, and for all practical considerations, the structure of the disc underneath the coating carries all loads generated by the applied pressure.
Similarly, US 2001/0011471 discloses a liner covering and sealing an opening in a rupturable portion of a rupture disc. In US 2001/0011471 the liner is said to be lighter and more flexible than the material of the rupture disc. It is not apparent that this liner has any mechanical strength.
The disadvantage of these coatings or liners is that, as they are thin, they are liable to mechanical damage and pin-holing. Any pin-holing of the coating allows the corrosive fluid to contact the structural part of the disc and gradually attack it, lowering its strength. This is a significant weakness with this type of design.
Attempts have also been made to produce twin disc assemblies for use in pressurised systems containing corrosive fluids. The present inventors have now analysed these as follows. For example, U.S. Pat. No. 4,591,520 discloses a twin disc assembly in which the primary disc, which is in contact with the high pressure side of the system, is made of graphite and is backed up by a supporting metal disc. However, such a system is expensive, due to the requirement for a special form of graphite that has a great strength and reliability. The two discs forming the assembly must also be shaped using different procedures, and thus it is not possible to produce discs that conform exactly. This lack of conformity means that the discs do not operate sympathetically under pressure. In particular, wrinkling or creasing of one of the discs may occur when the discs form part of a reverse-acting assembly. Thus, the reliability of the assembly is impaired. The reliability of the assembly is also impaired by the fact that it is not possible to introduce regions of weakness in graphite in a dependable manner. Furthermore, the mechanical failure of graphite is in general less predictable than that of materials more commonly used in the manufacture of bursting discs.
Twin disc assemblies comprising two metal bursting discs are also known. However, the examples disclosed in prior art documents such as U.S. Pat. No. 6,959,828 and EP0867648 are unsuited for use in pressurised systems containing corrosive fluids, since the discs in these assemblies contain through-thickness slots or cuts and thus allow corrosive fluid to penetrate into the assembly. Furthermore, neither of these documents discloses any means for ensuring that the discs operate sympathetically under pressure. Sympathetic operation of the discs is particularly important when the discs form part of a reverse-acting assembly, since any wrinkling or creasing of the discs reduces the reliability of the assembly.